As Sweden begins decommissioning its nuclear power plants, time is running out to find a way to make 9,000 tonnes of spent nuclear fuel safe for the next 100,000 years.

The nuclear industry says it has the answer, but environmentalists dismiss it as old and unsafe technology.

A 1980 referendum held in the country decided nuclear power should be phased out. The first reactor came offline in 1999, the second this week.

The remaining 10 reactors will all be shut down in the next few years, bringing to an end 40 years of nuclear history.

'Safe for 100,000 years'

Some 60m under the sea outside the Forsmark nuclear power plant just north of Stockholm, I am shown into a complex network of tunnels.

This is where contaminated equipment and clothing from the nearby power plant is stored. But it is also a showroom for what the industry hopes can be a final solution for a much bigger problem: the highly radioactive spent fuel.

Kai Ahlbom heads the geological research of the bedrock here, which he thinks would be suitable for permanent storage of the world's most toxic waste.

"This rock is 1,800 million years old. Not much has happened to this bedrock during that time," Mr Ahlbom explains. He is confident this geology will not change much for at least another 100,000 years.

That is how long spent nuclear fuel remains dangerous to the environment. It is the responsibility of the nuclear power plant operators here to make sure their waste remains safe until it is no longer radioactive.

Digging it down

The plan is to construct a deposit some 500m underground, where the fuel can be permanently stored. Today, spent nuclear fuel sits in temporary storage in the south of the country.

Kai Ahlbom believes disposal can be done safely

"We will encase the waste in 5cm-thick copper canisters, to protect against corrosion," Mr Ahlbom says.

"Then, we want to encase the cylinders in bentonite clay. It's basically like cat sand; it absorbs humidity very efficiently, and swells when wet."

After all nuclear waste has been stored, the site would be filled in, and safe enough to be left without human intervention until the radiation risk has gone, Mr Ahlbom believes.

'Old technology'

But environmentalists are not happy with the solution. Kenneth Gunnarsson, from the Swedish NGO Office for Nuclear Waste Review, told the BBC News website the waste problem was far from being solved.

"No one in the world has a solution. And the Swedish nuclear industry's solution is an old one they came up with in the 1970s. This is old technology," he says.

The president of Sweden's Society for Nature Conservation, Mikael Karlsson, agrees, and says the industry for too long has concentrated on one solution, and has made compromises on safety when its model has run into problems.

"Swedish legislation requires an assessment of alternative methods and locations, and that is something which the operators have not conducted yet.

"So they won't get any permits from the government for storing the waste according to their present proposals, if the legislation is to be followed," he said.

But while environmentalists are critical of the industry's failure to come up with alternative storage solutions, they have yet to present any alternatives themselves. And time is running out.

The temporary storage for spent nuclear fuel was designed to operate for 40 years. It is already half way through its lifespan.